The invention relates to fused silica and articles made therefrom. More particularly, the invention relates to fused silica having low concentrations of hydroxyl (OH) groups and deuteroxyl (OD) groups. Even more particularly, the invention relates to a method of making fused silica glass having low concentrations of the above-mentioned species in a halogen-free environment.
Fused silica optical components used in the semiconductor field, particularly in the area of photolithography, have stringent requirements for both dynamic and static properties. Such components are typically exposed to ultraviolet light having a wavelength of about 360 nm or less such as, for example, an excimer laser beam or some other ultraviolet laser beam. As the photon energy, pulse energy, and pulse rate increase, the intrinsic and extrinsic properties of the optical components such as density, refractive index, induced absorption, fluence dependent transmission (FDT), and laser induced wave front distortion (LIWFD), and polarization induced birefringence (PIB) are affected.
It has been generally known that high purity fused silica prepared by such methods as flame hydrolysis, CVD-soot remelting processes, plasma CVD processes, electrical fusing of quartz crystal powder, and other methods is susceptible to laser damage. The presence of hydroxyl (referring here to hydrogen having a naturally occurring isotopic distribution) groups in the silica increases resistance to optical deterioration. In addition, hydroxyl content may be controlled by ‘drying ’the fused silica soot blank with halogens, such as fluorides or chlorides. One approach to making fused silica for optical components is to form a silica soot containing no chlorine. The soot is doped with water and heated for a very long time to achieve a homogenous OH distribution throughout the fused silica article. In a second approach, the soot is processed such that the fused silica simultaneously contains low levels of water and chlorine. While low levels of these dopants help promote homogeneity, the distribution of chlorine and OH within the fused silica is difficult to control.